This invention relates to measuring the length of a run of cable, for example, the measurement of a length of cable from a selected point intermediate the two ends of the cable to a distant point, such as one end of the cable. Specifically, this invention is directed to the field of logging in boreholes, such as oil and gas wells, by means of a logging tool, or sonde, lowered on an armored multi-conductor cable which is longer than the depth of the borehole, where the length of cable introduced into the borehole is measured in order to ascertain the depth of the sonde.
Techniques for electrically measuring the total length, or the length of cable between one end and a break, or short, in a long conductor, such as a telephone cable and the like, are known. One technique for electrically measuring the length of cable from one end of a cable to the other, or from one end of a cable to a point where a conductor is broken, or where a conductor is shorted, is to make electrical contact with and apply a very short time duration electrical signal to one end of the conductor in question and monitor the time during which the electrical signal travels along the conductor to the other end, or to the break, or to the short, and is reflected on the conductor back to the end of the conductor to which the electrical signal was applied. The distance can then be computed based on the velocity of propagation of the electrical signal in the conductor. Such a technique, for example, is employed in time domain reflectometer cable testers, such as sold by Tektronix, Inc., of Beaverton, Oreg., under the designations Models 1502 and 1503.
The problem addressed by this invention is somewhat different. The problem is not to measure the total length of a cable, or the length of cable between one end and a break, or a short, in that cable, but rather to measure only a portion of the length of a cable, that is, between a selected first point, intermediate the two ends, and a second point, at a distant end, for example, under circumstances where it is neither desirable nor practical to make electrical contact with the cable at either the first or the second point. This invention provides the advantage of measuring a length of cable between the first and second points without electrically contacting the cable at either point.
Additionally, the known technique for measuring a length of cable between the mouth of a borehole and a sonde, and thus the depth of the sonde, is by means of a measuring wheel which is positioned at the mouth of the borehole. The cable is trained over the wheel so that the wheel rotates as the cable is introduced into the borehole. The wheel is precisely constructed, and the circumference of the wheel is very accurately known. The length of cable which has been introduced into the borehole can be measured as the cable enters the borehole based on a count of the rotations, or partial rotations, of the wheel so that the depth of the sonde is known.
However, because of friction in the wheel, slippage of the cable over the wheel, or faulty contacts in the slip rings of the counter circuit which is operated by the rotations of the wheel, there can be an error involved in the measurement of the length of cable introduced into the borehole, and thus an error in the measurement of the depth of the sonde. When an error occurs, the normal procedure is to raise the sonde back to the surface, reset the counter to zero, and reinitiate borehole logging by lowering the sonde into the borehole while again monitoring the operation of the counter. The loss of time and the expense of having to repeat the logging process is highly undesirable.